A Hg(00C-CH3)2 

 • HggClCHCOO" 



D HgClCeH+COO" 



10 20 30 A 



I I I u I I M I I I I I I I 



Fig. 1 . Positions of mercury atoms in different haemoglobin derivatives. Tlie /7-chloro- 

 mercuribenzoate and dimercuriacetate groups are attached to the sulphydryl groups. The 

 mercuri di-acetate groups are attached to a site whose chemical character is not known. 



ELECTRON DENSITY OF HAEMOGLOBIN MOLECULE 

 PROJECTED ON b PLANE : CONTOURS OF HIGH 



DENSITY REGIONS 



? 



10 

 I I I I I I I I I I 



50 A 



Fig. 2. Electron density distribution in the haemoglobin molecule seen in projection 

 along the dyad axis at a resolution of 2-8 Â. The black dots represent sulphydryl groups. 

 Contours are drawn at intervals of 2-5 electrons/A^ above the level of the suspension 

 medium, which is 2m ammonium sulphate solution. For the sake of clarity the three 

 lowest contours have been omitted, and only the peaks are shown. The contours near 

 the periphery of the molecule are not exact, being partly modified by the overlapping of 

 neighbouring molecules in the crystal lattice. 



